Oligosaccharide synthesis by selective

Oligosaccharide Synthesis by Selective Anomeric Activation with MOP- and TOPCAT-Leaving Groups... 

B. Lou, E. Eckhardt, and S. Hanessian, Oligosaccharide synthesis by selective anomeric activation with MOP- and TOPCAT-leaving groups, in S. Hanessian, (Ed.), Preparative Carbohydrate Chemistry, Marcel Dekker, Inc., New York, 1997, p. 449. 

This procedure has been demonstrated to provide moderate yields and anomeric selectivity in oligosaccharide synthesis. For instance, the disaccharide 110 was obtained in 50% yield as a 1 2 a p ratio. The reaction side products were mainly the self-condensed donor (10-25%) and unreacted hemiacetal (5-10% or higher). Alternatively, the a-linked glycosides were favored with diethyl ether solvent. In this way, trisaccharide 111 was prepared from the disaccharide hemiacetal donor in 49% yield, favoring the a-anomer by 4 1. 

The base lability of succinoyl diester hnker severely limits the selection of protecting groups available for an oligosaccharide synthesis, so a more versatile tether was required. Diether bonds of benzylphenol or dibenzyl of 1,4-di(hydroxymethyl)-benzene satisfy this requirement because they are stable to both bases and to acids. A sufficient acid stability is important since the formation of a glycosidic bond is an acid-catalyzed reaction, not surprisingly, as it is an acetal functionality. For instance, DOX,34 the dibenzyl hnker a,a -DiOxyXylyl diether, -0CH2C6H4CH20-, is not limited by restriction of the succinoyl hnker (1) when bound via a hydroxyl or as an... 

Acid-catalysed alkylation of an alcohol with O-alkyl trichloroacetimidate prepared from allyl alcohol and trichloroacetonitrile is readily accomplished Scheme 4.233]440 as previously discussed for the preparation of benzyl and tert-butyl ethers.311 However, these conditions are not compatible with many of the protecting groups employed in oligosaccharide synthesis. For such cases, two methods for 0-allylation under essentially neutral conditions have been devised. The first method takes advantage of the mild conditions and regioselectivity of stannylene alkylations (see section 4.3.3). The method is illustrated by the selective O-allylation of o-lactal, which began with stannylene formation on an 0.8 mole scale [Scheme 4.234].441... 

---